Workpieces, such as vehicle parts, for example, are provided for purposes of optics, decoration, and inhibition of corrosion and wear, with a coating.
Conventional coating methods are chroming, vaporizing or painting.
In the case of chroming, especially bright-chroming, materials of iron, copper, copper-zinc, copper-tin, and aluminum are conventionally used. However, the workpiece surface to be chromed often requires manual or mechanical polishing.
The thermal vaporizing (vapor deposition, vapor coating) that is also used for the coating of metal surfaces is a high-vacuum-based coating technique that comes under the heading of the PVD processes. Typical materials for this operation are metals, such as aluminum, copper, silver, and gold, for example, but also other suitable materials. This known coating method, however, necessitates a relatively high level of technical complexity.
For vehicle finishing, especially rim finishing, metallic paints are used with particular advantage.
Paints, in accordance with DIN 971-1 (09/1996), are coating materials in liquid, paste or powder form which, when applied to a substrate, produce a covering coating which has protective, decorative or specific technical properties. The principal components of the paints are binders, solvents, pigments, fillers, and paint auxiliaries. These paints are subdivided according to various technical standpoints.
Metallic paints are effect paints having the typical characteristics of a metallic gloss, a high light/dark flop, and high brightness, and also high hiding power. Metallic finishes. They comprise platelet-shaped metallic pigments which, in application, adopt an orientation largely parallel to the substrate. In this way, by the action of numerous small mirrors, a directed reflection is produced that is responsible for the metallic gloss. However, there is also always a certain fraction of scattered radiation produced, which comes about as a result of incomplete plane-parallel orientation of the pigments and also as a result of roughnesses in the individual pigment particles, such as edges or uneven surfaces, for example. Metallic finishes are therefore characterized by this interplay of directed and diffusely scattered radiation, which produce the light/dark flop. Also, in the case of common metallic finishes, although it is no longer possible to discern individual pigment particles, it is nevertheless always possible to discern a particulate texture to the paint. This phenomenon is difficult to detect by colorimetry.
Commonly, therefore, the visual appearance of metallic paints pigmented with platelet-shaped metallic pigments is different from the aspect of pure metal surfaces, where there are virtually no scattering effects.
Metallic paints are also obtainable through use of expensive PVD metallic pigments.
The PVD pigments, whose preparation is relatively expensive, are nonleafing pigments, i.e., pigments which are completely wetted by the binder and which are distributed throughout the paint film, but not at the surface of the paint film. PVD pigment paints have a binder fraction of only up to about 10% by weight, based on the total weight of the paint, and so the optical paint properties of the pigment, such as gloss, for example, are manifested in full.
In order to obtain the desired finishing effects with a metallic paint comprising PVD metallic pigment, the substrates to be painted must be very smooth (even) and thoroughly pretreated. It is disadvantageous, furthermore, that a workpiece bearing abrasion scars cannot be painted with a metallic paint comprising PVD metallic pigment, since the abrasion scars are not hidden. In addition, it is not possible to eliminate finishing defects. Instead, in the case of a finishing defect, the PVD finish must be removed from the workpiece and the workpiece must be painted anew.
Furthermore, these known chrome effect paints comprising PVD pigments have a tendency, particularly in the case of a sprayed finish, to form “mottle”, i.e., areas of shading which have different lightnesses and darknesses, in the paint surface, thereby adversely affecting the optical qualities of the paint surface. The mottling is the result of the nonuniform orientation of the conventional aluminum pigments, particularly through aluminum pigments which stand almost vertically in the finish.
These finishing defects are recorded in particular in the finishing of materials of large surface area, such as of bodywork parts, for example. Moreover, an optimum finish effect is dependent on the parameters of the particular mode of application (dry, wet).
A further disadvantage of PVD pigment paints is their problematic management in aqueous coating compositions. On account of their extremely high and smooth specific surface area, their tendency toward agglomeration is very high. Moreover, PVD pigment paints are relatively difficult to apply. Automobile finishes with a uniform appearance can be accomplished only by means of manual painting in thin coats in numerous painting steps. Such sensitive application behavior runs counter to the kind of automated painting which is usual, for example, for vehicle finishes.
Owing to high interface surface tension, leafing pigments are not wetted by the binder, and therefore float in the aqueous paint film and become oriented at the paint film surface. This effect is obtained, for example, through the use of stearic acid as a grinding assistant in the milling of particles of a metal shot, such as of an aluminum spray shot, for example. Platelet-shaped leafing pigments of this kind form a dense mirror at the paint surface, composed of metallic effect pigments oriented parallel to the paint surface. These metallic effect pigments, with a high metallic gloss, however, have only limited wipe resistance and scratch resistance, since the metallic effect pigments are inadequately fixed in the binder matrix. As a result of their floating behavior, therefore, leafing pigments produce a dense barrier layer at the paint surface. Consequently, effective capacity for recoating, such as with a clearcoat, for example, is no longer possible. This barrier layer hinders or prevents reliable adhesion of the topcoat to the basecoat surface, and so, in the course of use of a painted article, there may be instances of detachment of the topcoat film and of damage to the coat containing metallic effect pigment. Moreover, the topcoat, a clearcoat, for example, adversely affects the optical properties of the metallic basecoat, especially its gloss, i.e., the gloss is reduced.
A further substantial disadvantage of conventional metallic finishes is their wipe resistance and scratch resistance, which are not adequate for every end use, and also their abrasion stability (DIN 55923), owing to the above-described inadequate fixing of the aluminum flakes in the binder matrix.
When aluminum effect pigments are used in environment-friendly aqueous paint systems, such as water-based paints, for example, there is the problem of preventing a chemical reaction of aluminum with water, to avoid unwanted evolution of hydrogen. The conventional hydrophobing of the aluminum surface that is employed in response to this problem, by means of grinding assistants, such as stearic acid, is not usually enough to prevent the reaction of the aluminum pigments with water and hence the loss of metallic gloss and adverse effect on storage stability, through agglomeration, for example. For aqueous paint systems, therefore, water-based paints, for example, aluminum effect pigments used are provided with an anticorrosion coating. The anticorrosion effect is produced, for example, by the application to the aluminum surface of corrosion inhibitors, organophosphorus compounds for example. Furthermore, the aluminum pigments may also be passivated, i.e., protected from corrosion, by what are called conversion coats, such as by chromating (EP 0 259 592 B1), for example. A third stabilization principle is based on the complete encapsulation of the aluminum effect pigment in a chemically inert, largely transparent coat, typically protective coats produced by sol/gel operations, such as an SiO2 coating, for example.
For the pigmentation of high-gloss metallic paints it is advantageous to use thin, platelet-shaped, leafing aluminum effect pigments which are obtained by mill-shaping of aluminum shot, and which are also referred to as “silver dollars”. These aluminum effect pigments—in comparison to the aluminum pigments from comminution milling that are referred to as “cornflakes”—have a relatively round form and a relatively smooth surface. The aluminum effect pigments that are also referred to as cornflakes differ from the aluminum effect pigments that are also referred to as silver dollars in that the surface is rougher and some of the edges have indentations.
WO 01/81483 relates to a pigment preparation and to an aqueous effect basecoat produced therefrom, with particularly good shearing stability, for vehicle finishes. The pigment preparation comprises at least one carboxyl-functional resin and water-miscible organic solvents, and also metal pigments coated with a silicon-oxygen matrix, examples being aluminum pigments such as commercially available “STAPA IL Hydrolan”, a nonleafing aluminum effect pigment from Eckart. The paint compositions described therein correspond to a typical metallic paint, but without having the special features of a chrome-effect paint.
Furthermore, WO2006/110331A1 discloses a coating composition for corrosion control paints for metal, plastic, and other substrates. These compositions include metal pigment powders (in flake form and also as metal powders) having a grain size of 100-325 mesh. According to the U.S. Bureau of Standards, this relates to pigments having a D100 of approximately 212 μm and a D50 of approximately 62 μm.
DE 100 39 404 A1 relates to an organically modified, inorganic pigmented composition for corrosion control on metal surfaces. This composition, prepared by means of a sol-gel process on the basis of polysiloxanes, may also be pigmented with leafing metallic pigments that are prepared by conventional milling of circular aluminum grains in ball mills in the presence of “lubricants”, such as stearic acid, for example, and have an average particle diameter of at least 0.5 μm.
Contrastingly DE 198 20 112 A1 discloses an effect pigment which is coated with at least one reactive surface modifier and is intended for preparing inks, printing-inks, paints, coatings, and plastics. These effect pigments, which have a particle size of 1 to 200 μm include aluminum effect pigments among others, are said not only to be readily wetted by the binder or solvent of the ink or paint but also to undergo effective orientation in the liquid paint film and, moreover, to enter with their surrounding binder matrix into an intimate bond, in order to enhance properties of the application medium, such as weathering stability, corrosion control, brilliance, and impact strength, for example.
Known from DE 263 07 31 B2 is the use of leaflet-shaped nonleafing and leafing aluminum pigments with a translucent polysiloxane coating in electrostatically sprayable wet paints. This wet paint, which can be applied exclusively by electrostatic means, is prepared by incorporating the silane compound, in solution in a solvent, into a paste of the metallic pigments, and at the same time initiating the hydrolysis in a known manner. The hydrolysis and/or crosslinking of the silanes is accomplished chemically or thermally.
DE 10100195 A1 relates to an aqueous effect coating material comprising effect pigments, binder, and a neutralizing mixture of at least two fatty acids. In this coating material, whose possible uses include motor vehicle finishing, the pigments that can be used include passivated silver dollar and cornflake aluminum effect pigments having a particle diameter which is characteristic of these pigment types.
WO 2005/118722, moreover, discloses an aqueous coating composition comprising at least one water-compatible film-forming agent and platelet-shaped aluminum pigments that have at least one inorganic corrosion control coat. The passivated aluminum effect pigments present in this coating composition, which can also be used for high-gloss automotive finishes, the pigments being prepared by mechanical shaping of spray shot, have an average thickness of at least 50 nm. In application, the coating composition described here does not have the gloss of a chrome-effect paint.
DE 697 06 471 T2 relates to a water-thinnable coating composition which can be applied by heat-curing to a substrate, including a metal substrate such as steel. This coating is used as a corrosion control coating and its components include a high-boiling organic liquid (boiling point above 100° C.), a particulate metal, such as aluminum flakes, for example, and a water-reducible epoxy-functional silane binder.
DE 20 2006 016 073U1 discloses a chrome-effect paint and a chrome-effect finish for motor vehicles and motor-vehicle parts. This chrome-effect paint comprises a suspension of nonleafing aluminum pigments having an average particle size of less than or equal to 10 μm, and at least one organic solvent, such as butylglycol, for example. The claimed chrome-effect finish consists of at least one undercoat, an effect coat comprising the aluminum pigments, and also a clearcoat.
DE 101 54 030 A1 discloses an aqueous effect coating composition which is to be useful for applications including finishes on motor vehicle bodywork/parts. This known coating material comprises different binders, such as (meth)acrylate co(polymers) and polyurethanes, for example, effect pigments, and silicon compounds that take on the function of a stabilizer for the effect pigments and the function of the crosslinking agent. Effect pigments specified include a multiplicity of pigment types in different use forms, in the form of organic and inorganic coloring pigments, metal flake pigments, and nonmetallic effect pigments, for example. In the only example to illustrate the subject matter of the application, however, an aluminum effect pigment was named, but without further physical characterization.
DE 696 25 797 T2 discloses a curable resin composition for use in water-based coating materials. The coating materials produced using the new resin composition, and intended, among other applications, for use in the automotive segment, for example, are said to have substantially improved coating properties and curing properties, and also excellent resistance to weathering, solvents, chemicals, and water. This known coating material (resin composition) comprises at least one emulsion polymer (A) having tertiary amino groups and a compound (B) having at least one epoxy group and hydrolyzable silyl group, and may also comprise a further compound (C). In order to obtain a metallically lustrous surface, this coating material may also comprise an aluminum paste, of which no more detailed description is given.
DE 10 2005 026 523 A1 relates to a two-component anticorrosion paint comprising metal pigment, epoxy binder component, and aminic hardener, for producing corrosion control coatings. This known protective paint is composed of a component (A) having platelet-like metal pigments and at least one epoxysilane and/or epoxy silicone, and also an organic solvent, and a component (B) having at least one aminic hardener and platelet-like metal pigments and an organic solvent. In this known anticorrosion paint, platelet-like metallic pigments containing zinc and aluminum are used in different weight proportions and mixing ratios. With this paint, the optical-decorative properties are not of interest.
Finally, EP 0 451 785 B1 presents nonleafing aluminum pigments which are prepared by conventional wet milling and have a high reflectance and high opacity. The average particle diameter of the aluminum pigment, which is passivated by a covering film, is 5-25 μm.